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 DRAG REDUCTION IN SHIPS USING MICROBUBBLES TECHNOLOGY full report
Post: #1

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ABSTRACT
Drag is a mechanical force generated by a solid object moving through a fluid. For drag to be generated, the solid body must be in contact with the fluid. Drag is generated by the difference in velocity between the solid object and the fluid. Drag acts in the direction opposite to the direction of motion of the body.
Microbubbles is a drag reduction device that reduces skin friction of a solid body moving in water by injecting small bubbles into the turbulent boundary layer developing on the solid body.
Itâ„¢s application is confined to the ships, especially large ships. Ships such as tankers play a major role in marine transportation. They are very large and move very slowly. They are especially suited to microbubbles application. With the development of the microbubbles technology almost 20-80% drag reduction is possible
Introduction
Drag is a mechanical force generated by a solid object moving through a fluid. For drag to be generated, the solid body must be in contact with the fluid. If there is no fluid, there is no drag. Drag is generated by the difference in velocity between the solid object and the fluid. There must be motion between the object and the fluid. If there is no motion, there is no drag. Drag acts in the direction opposite to the direction of motion of the body.
A body moving through a fluid experiences a drag force, which is usually divided into two components: frictional drag (sometimes called viscous drag) and pressure drag (sometimes called form drag or profile drag).
Frictional drag comes from friction between the fluid and the surfaces over which it is flowing. This friction is associated with the development of boundary layers. Pressure drag comes from the eddying motions that are set up in the fluid by the passage of the body. This drag is associated with the formation of a wake, which can be readily seen behind a passing boat. When the drag is dominated by viscous drag, we say the body is streamlined, and when it is dominated by pressure drag, we say the body is bluff. Whether the flow is viscous-drag dominated or pressure-drag dominated depends entirely on the shape of the body. A streamlined body looks like a fish, or an airfoil at small angles of attack, whereas a bluff body looks like a brick, a cylinder, or airfoil at large angles of attack. For streamlined bodies, frictional drag is the dominant source of air resistance. For a bluff body, the dominant source of drag is pressure drag. For a given frontal area and velocity, a streamlined body will always have a lower resistance than a bluff body. For example, the drag of a cylinder of diameter \$D\$ can be ten times larger than a streamlined shape with the same thickness (see figure 1).
Microbubbles Technology is the latest development to the study of the drag reduction in ships. Almost 20-80% reduction is possible using this technology.
Why microbubbles
Microbubbles is a drag reduction device that reduces skin friction of a solid body moving in water by injecting small bubbles into the turbulent boundary layer developing on the solid body. But at the same time the energy needed for injecting bubbles at he hull bottom is not small because large ships have large water depth against which the bubbles have to be injected. Therefore it is important to reduce the amount of injected air in order to put microbubbles into practical use.
Fig.1 shows an example of its skin friction reduction effect. The data was a circulating water tunnel, where the bubbles were injected at the top flat wall and skin friction was measured by a skin friction sensor placed downstream of the injection point. The horizontal axis shows the amount of injected air and the vertical axis shows the ratio of reduced skin friction to that at non-bubble condition. This figure shows that, as the amount of injected air increases, skin friction reduction effect by microbubbles increases up to 80%.
Ships such as tankers play a major role in marine transportation. They are very large and move very slowly. They are especially suited to microbubbles. Fig.2 shows an image of the application of microbubbles to such a ship. One reason that they are suited is that their skin friction drag component occupies about 80% of the total drag. The drag of a ship that moves on the water consists of two components, i.e., wave-making drag and skin frictional drag.
The wave-making drag component of such a ship is very small because they move very slowly. Another reason that they are suited is in their shape. Their shape is like a box, except for bow and stern regions. They have a wide flat bottom, and the bubbles injected at the bottom near the bow stay close to the hull bottom by buoyancy while they are carried by flow all the way to the stern. Thus the injected bubbles can cover the whole hull bottom efficiently. In Japan, microbubbles have been studied intensively in the past few years toward its application to full-scale ships. This paper reports a review of such studies, focusing on those carried out by our group in NMRIJ.
Mechanism and Scale Effects of Skin friction Reduction by Microbubbles
Studies on microbubbles were carried out, aiming at its application to full scale ships. In the basic studies using a circulating water tunnel, it was found out that the local void ratio near the solid wall is important for skin friction reduction.
Experiments using a 50m-long flat plate ship were carried out in a towing tank. Bubbles were injected at two stream wise locations to find out the effect of the boundary layer thickness. It was found out that the boundary layer thickness has little effect and that the distance from the injection point is the most important factor.
Experimental apparatus
The experiments were carried out using the high speed circulation tunnel as shown in Fig.1. The test section has the following inner dimensions, the width 100mm, the height 15mm, and the length 3000mm.
Fig.2 shows the detail of the air injection chamber. Air was injected into the flow plate through a plate with many regularly-spaced 1.0mm diameter holes called an array-of-holes plate shown in Fig.3. The width and length of the injection part was 72mm, and the pitch of the holes are 2.5mm in the stream wise direction and 1.8875mm in the span wise direction, resulting in the total of 277 holes. It was set at the position of 1028mm from the inlet of the test section.
The bubbles produced by the array-of â€œholed plates were about 1mm in diameter. The local skin friction was measured directly using the shear sensor shown in Fig.4, whose capacity was 2gf and diameter is 10mm.
Photographs of the microbubbles was taken using the setup shown in Fig.5. A YAG laser was used as a light source, whose light sheet penetrates the test section at the position 30mm from the side wall, where a CCD camera is placed outside.
The relation between void ratio and skin friction reduction
The skin friction was measured at three speeds in three stream wise locations. The results are shown in Fig.6. The horizontal axis shows the average void ratio in the test section defined as
where Qa : volumetric flow rate of air in the test section
Qw : volumetric flow rate of water in the whole test section of
100mmÃƒâ€”15mm
At all the three speeds, the skin friction reduction as the amount of injected air increases. At the average flow speed V=5m/sec, measured values at three locations agree well with each other, and agree with the average curve of measurements, which is shown as
where is the average void ratio in the test section. At V=7m/sec, measured values at three locations are different from each other, they consistently deviate from the Merkleâ„¢s curve. The local void ratio was measured using a suction tube system. The measured results, which have been adjusted so that the air volume integrated height agrees with that measured at the injection point, are shown in Fig.7.
The scale effect of microbubbles
In order to be able to estimate skin friction reduction effect of microbubbles when it is applied to a full-scale ship, it is necessary to carry out large-scale experiments and find out how far in the downstream direction the skin friction reduction effect of microbubbles persists after injection. Watanabe measured the reduction effect by using a flat plate ship of 40m long and 60cm wide in a towing tank. The authors carried out a similar experiment using a flat plate ship of 50m long and 1m wide in their 400m-long towing tank. Fig.8 shows the flat plate ship that they used, and Fig.9 shows the distribution of skin friction reduction in the stream wise direction. It is seen that the skin friction reduction effect persists to the downstream end at both speeds of U=5m/sec and 7m/sec, which is a good sign for its application to a large ship.
Fig.8 50m long flat plate ship for microbubbles experiment
The skin friction reduction effect is smaller at the higher speed of 7m/sec, whose reason may be that injected bubbles diffuse from the solid wall faster at higher speeds. Further study is needed. Watanabe obtained higher skin friction reduction at U=7m/sec than the authors. One of the main difference between the two is that they used a porous plate for bubble injection and the authors used an array-of-holes plate, which has many 1mm diameter holes arranged in arrays. Further study on this point may lead to improvement in skin friction efficiency.
(a)Total drag (b) Integrated skin friction
downstream of injected plate
Fig.9 Drag reduction (Air injection at bow)
.
Various Methods of Bubble Generation
I. Venturi tube type bubble generator
This bubble generator has a very simple structure as shown in Fig. 10(a). Air is injected at upstream side of the throat. As the mixture of the air and water passes the nozzle throat, the bubbles grow due to the pressure decreases caused by the increase in the velocity. Then the bubbles collapse in the diverging part of the nozzle because of the recovery of the pressure. Due to the decrease in the sonic speed in the bubbly flow, the flow velocity exceeds the sonic speed. This forms a shock wave in the diverging region, and the bubbles are supposed to experience
very steep pressure recovery.

The performance of this Venturi tube type bubble generator was evaluated for ranges of air volume fraction and liquid flow rate. Fig. 10(b) shows the relation between the void fraction and the bubble size distribution. It is shown that bubble size distribution is quite independent of the void fraction up to = 20%. The arithmetic mean diameter is about 0.1 mm. The symbol Ds in the figure denotes the area equivalent diameter defined as
Fig. 10© shows that bubble size distribution changes when the liquid flow rate is small. This is due to the different collapsing behavior at a low and a high liquid flow rate.
As shown in Fig. 10(d), at the liquid flow rate Qw = 6.7 l/min. bubbles collapse suddenly, while change in the diameter is slower at Qw = 4.2 l/min. The simple average velocity V at the throat is 9.9 m/s at Qw= 4.2 l/min. and 15.8 m/s at Qw- 6.7 l/min. The corresponding cavitation number s = (P8 â€œ Pv)/0.5V2 is about 2.0 and 0.80 respectively. It is supposed that the intensive collapse of bubbles occur when cavitation number s <1.0.
II. Tangential Water-jet
The diameter of bubbles generated by injecting air into a turbulent boundary layer through array of holes or porous medium depends on the mean shear stress at the wall. Therefore, one way to control the bubble diameter is to increase or decrease the local mean wear stress at the wall where air is injected. This is achieved by two different methods in this study. The first one
described in this section uses a tangential water jet to increase the local shear stress on the plate with array of holes through which air is injected. Fig.1 shows the set up of the experiment carried out at National Maritime Research Institute, and Fig.11 shows a close view of the tangential water jet system. The inner diameter of the air injection holes is 0.5 mm. The channel is made of transparent acryl resin so that optical measurements and observation of bubbles are possible. The test section is 15 mm in height, 100 mm in width, and 3000 mm in length. The air injection chamber with the water jet system is attached to the top wall. The results for the bulk mean velocity of U=3 m/s, and the water jet flow rate of q=0 â€œ 30 l/min. are presented in this paper. The ratio of the water jet flow rate to that of the main flow is 1:9 at Q=30 l/min.
III. Foaming of dissolved air
The previous investigations have shown that the influence of the bubble size on the drag reduction is negligibly small within the investigated range of bubble diameter. However, there is still a prospect that bubbles that are sufficiently small compared to the characteristic scale of turbulence may significantly influence the drag reduction rate. Therefore, we investigated a method to generate bubbles of 20 Ã‚Âµm â€œ 40 Ã‚Âµm in diameter; Bubbles of these sizes are utilized for mixing a separation processes and are usually generated by foaming microbubbles from air dissolved in liquid. Since the microbubble drag reduction requires higher void ratio than other applications such as mixing or separation, we investigated whether this method is applicable to the purpose of drag reduction.
Fig.11(a) shows the test section used in the experiment carried out at University of Tokyo. The test section is 120Ãƒâ€”50Ãƒâ€”580 in height, width and length respectively. Sufficiently aerated water under the absolute pressure of P1 = 0.8MPa in a pressure tank is
introduced into the test section through a slit.
The water from the pressure tank is depressurized to the absolute pressure level inside the test section Po = 0.1MPa at the valve installed before the slit. Since the solubility of air in water is proportional to the pressure, the excess air is separated and makes microbubbles. The estimated void ratio obtained by this procedure is given by

in which C is solubility of air in water (cm3/cm3), which is 0.02 under P0 = 0.1MPa at the temperature of 20o C. The void ratio is estimated to be about 0.12 for P1 = 0.8MPa.
In order to measure the diameter of bubbles, the depressurized foaming water was introduced to a thin channel to which a microscope was mounted. Fig. 12 shows an example of picture through microscope. The bubble diameter measured on the pictures by hand is shown in a histogram in Fig. 13. The shape of the distribution is similar to that shown in Fig. 8 for bubbles generated by shear stress, but the range of the diameter is an order of magnitude smaller. The most frequent diameter range is between 20 and 40 Ã‚Âµm, which includes one out of two bubbles, and the calculated average diameter was 47 Ã‚Âµm. The void ratio estimated from the bubble number distribution and the depth of the picture was about 5% which is on the same order as the value estimated from the solubility of air in water.
This foaming water is introduced into the turbulent boundary layer inside the test section through a slit at the rate of Q = 5 â€œ 15 l/min. Fig. 14(a) shows a picture of bubbles in the test section near the injection point at the free stream velocity of U = 1.5 m/s. and the flow rate Q = 10 l/min. A picture of bubbles generated by injecting air through a porous plate at the same free stream velocity is shown in Fig. 14(b) for comparison. The bubbles generated by the present method look more like cloud or smoke. Due to the limitation of the apparatus, it was impossible to measure the bubble diameter distribution accurately in the test section. However, the appearance of bubbles was very similar to that in the channel for microphotography suggesting that the bubble diameter distribution is also similar. It was also noted that the appearance of the bubble cloud was not dependent on the free stream velocity.
Frictional drag reduction by microbubbles
i. Total drag
The measured total drag of the 12m flat plate ship in the non -bubble condition is shown in Fig.15. Using Prohasakaâ„¢s method with n=4 and using the data below Fn = 0.15, the form factor has been determined to be 1+K=1.1678.
The measured total drag of the 50m flat plate ship in the non-bubble condition is shown in Fig.16. The two tests carried out in 1999 and 2000 gave almost the same result. At very low speed the data in 1999 show laminar speed.
ii. Reduction of frictional drag by microbubbles
The reduction of total drag by microbubbles was measured and the reduction was attributed to the reduction of the frictional drag component Rf. Rf0, the frictional drag of the area Sb (Fig 17)m which is downstream of the air injection plate, in the non-bubble condition of the air injection plate was estimated by Scoenherrâ„¢s formula.
The result is shown in Fig.18 for the 12m flat plate ship of NMRIJ, and in the Fig.19 for the 50m ship of the NMRIJ. The horizontal axis shows qb, the injected air rate non-dimensionalized by shipâ„¢s speed and Sb. It is seen that the reduction is smaller at higher speed and that the reduction per unit qb is much higher with the 50m ship than the 12m wide ship. The reason for the latter may be that the skin friction reduction effect of microbubbles persists for a long distance downstream.
Application of microbubbles in ships
First an equation for the net power saving of a full scale ship is derived by considering the skin friction reduction by microbubbles and the power needed to inject them. Second, issues on the practical application of microbubbles to ships are discussed. Finally, the net power saving value is considered using available experimental results and typical specifications of a full scale ship.
1. Equations for net power saving
We assume that a ship runs at speed U with power W0, 0 denoting the non-bubble condition, and by injecting bubbles the ship runs at the same speed U with different, hopefully reduced, power W.
where D0, D: shipâ„¢s drag. (â€œD) and (â€œW) the drag and power reductions, are related as
where
Let DF be the frictional drag and define
Let DFb be the frictional drag of the surface covered with bubbles. The ratio DFb / D should be approximately equal to
where
S: wetted surface area of the ship,
Sb: surface area covered with bubbles,
but by selecting the Sb location near the bow we can expect, using a parameter mb > 1.
Since the drag reduction occurs only in DFb,
In order to estimate the net drag reduction effect, one should subtract the power needed for bubble injection from the power gain due to drag reduction. The pumping power Wpump is expressed as
where
: water density,
g: gravity acceleration,
z: water depth at injection point,
p: dynamic pressure at injection point,
Q: volumetric air injection rate.
By introducing nondimensional parameters
Wpump is expressed as
Finally the net power saving ratio rnps is expressed as,
where D0 has been nondimensionalized as
For example, rnps =0.05 means 5% net power saving. It is the purpose of microbubbles studies to maximize this parameter.
2. Issues on the application of microbubbles to ships
a) 1 â€œ DFb / DFb0
This parameter represents the reduction of the frictional drag by microbubbles and therefore is the most important. It is not the local value but the integrated value over the entire surface covered with bubbles.
Fig. 34 shows the values for the 50m flat plate ship in the bow injection case. The frictional drag in that area to be covered with bubbles in the non-bubble condition was estimated using the Schoenherr experimental formula shown below.
The figure again shows that the skin friction reduction is smaller at higher speed. This fact suggests that microbubbles should be applied to slow speed ships. The maximum reduction at V=7m/sec reaches over 20%. The corresponding reduction in the total drag was 9%.
The reason for the higher decay of the skin friction reduction effect at the higher speed is suspected to be the higher diffusion of the bubbles away from the wall by the higher turbulence intensity and/or smaller bubble size.
b) rs
We should efficiently cover a wide surface area with
bubbles.
c) rF
The wave-making drag, the other major drag component, increases quadratically with speed, and therefore the smaller the speed, the greater rF becomes, and the better-suited to microbubbles. With a displacement type ship such as a large tanker at the cruising speed of 14knots (7m/sec), rF reaches 0.8.
d) CQ / CD0
We should minimize the air volume for a given skin friction reduction.
e) rz / Fn2
This ratio, being equal to gz/U2, means that the pumping power reduces quadratically with speed. So higher the speed, the better the performance. Note that the skin friction reduction effect decreases with speed at the same time. It would also be possible to reduce rz by utilizing the downward flow near the bow.
f) CP
By choosing the location for bubble injection or designing the local shape, one can reduce CP and thus improve the net power gain.
g) Hull form
As Fig.2 suggests, the hull form of a large tanker is regarded suitable for microbubbles, because itâ„¢s flat and very wide bottom will help the bubbles injected at the bow stay close to the bottom by buoyancy. It is also important to consider effects of non-horizontal surface, pressure gradient and surface curvature.
h) Sea water
Bubbles generated in sea water are in general smaller than those in fresh water with which almost all the laboratory experiments were carried out. This will affect the trajectories and the skin friction reduction effect.
i) Propeller performance in bubbly flow
As Fig.2 shows, there is a good chance that the bubbles go into a propeller operating at the stern end. Ichikawa and Matsumoto 1995, measured lift and drag of a 2D NNCA4412 wing section in various average void ratio (Fig.20), and found that lift decreased and drag increased by the presence of bubbles. Therefore, we should expect that the propeller performance suffers when bubbles go into the propeller, and we should avoid it.
The influence of inflowing bubbles on propeller vibration should also be investigated.
j) CFD prediction methods
In order to predict the drag reduction performance of bubbles at full scale, one must first simulate the flow around a full scale ship, which is already very difficult. Second, one must predict bubble trajectories by considering the bubbles as a group, which means the two-way coupling approach.
3. Estimation of rnps
rnps is now estimated. Observing the plots in Fig.21 are almost linear at 7 m/s.
let us define the slope
CF, the skin friction of a ship, is traditionally related to CFsch, the skin friction of a flat plate with the same length and area, as
where 1+K is called the form factor.
Using the parameter values shown in Table 1, in which dimentions are of a large tanker, the bubble area is the front half of the hull surface, mb is given as the ratio of CFsch of 50 m and 100 m. rF and 1+K are typical values of a large tanker. Using the values it results that
Which means 5% net power saving. If z can be reduced to 5m somehow, it becomes that rnps = 0.069.
Net Drag Reduction by Microbubbles
In order to assess the applicability of microbubbles to ships, it is necessary to discuss its net drag reduction effect taking into account the energy needed for injecting air bubbles into water. In this section, net drag reduction of microbubbles is estimated.
Net work ratio

The net work ratio rw, i.e., the net work rate needed to propel a ship while injecting to that with no bubbles, is
where
W0: work rate to propel a ship in non-bubble condition
Wnet: net work rate to propel a ship in bubble condition
D0: ship's drag in non-bubble condition
D: ship's drag in bubble condition
U8: ship's speed (Assumed to be unchanged by bubble injection)
Wpump: work rate for bubble injection
rw = 1.0 when net drag reduction is zero, and
rw< 1.0 when there is net drag reduction effect.
Wpump is expressed by taking into account the energy loss due to head pressure at injection point and the local pressure there.
where
QA: air flow rate for bubble injection
: water density
g: gravity acceleration
d: water depth at injection point
Cp: local pressure coefficient at injection point
Ship's drag D is expressed in a conventional non dimensional form.
where
S: wetted surface area of a ship
CT: total drag coefficient
CF: frictional drag coefficient
CW: wave drag coefficient
CF0: frictional drag coefficient of equivalent flat plate (having the same
area and length as the ship)
K: form factor of a ship hull
Schoenherr's empirical formula is used to estimate frictional drag of the equivalent flat plate.
where
Re is the Reynolds number.
the drag coefficient of a ship in bubble condition, is expressed as
where it is assumed that CW and K do not change with bubble injection.
Thus, the net work ratio rw is expressed as
where
is the ratio of wave drag to viscous drag.
is the Froude number based on water depth.
How to Increase Drag Reduction Effect
Using the equation derived for rw, we can say that following five points are important to increase skin friction reduction effect and thus decrease rw.
(1)Reduce rD, i.e., choose a hull form that has small wave drag.
(2)Reduce , i.e., increase skin friction reduction effect by
microbubbles.
(3)Reduce, i.e., reduce injected air flow rate.
(4)Increase, i.e., speed up and/or make draft shallower.
This is because the loss (work needed for air injection) is static (speed-independent), and the gain (skin friction reduction) is dynamic (proportional to speed squared).
(5)Reduce CP, i.e., inject air at a location that has low pressure.
Drag Reduction Estimation in case of Tanker
The net work ratio W r is estimated for a large tanker with 300m length and the speed of 14kts. Typical parameter values for this case are,
Ship length L = 300m, d = 20m (full load),
S = 0.24L2, U8 = 7m/ sec, K = 0.35,
Fd2 = 0.25, rD = 0.25, Cp = 0,
Re= 2.1Ãƒâ€”109, CF=0.0014
The relation between injected air flow rate QA and skin friction reduction effect is roughly estimated, using the data of a 50m-long flat plate ship shown in Table 1. First, the skin friction reduction effect of a full-scale ship is assumed to be = 0.77, the same value as that of the 50m-long flat plate ship. Then QAS ( S means "ship"), the air flow rate in full scale is estimated by multiplying
QAM (M means "model") = 0.04Ãƒâ€” 0.05Ãƒâ€” 7 = 0.014m3 / s , the air injection rate at 50m-long flat plate ship, with the area ratio
to be QAS = 12.1 m3 / s . In this case, using equations for the net work ratio, the net work ratio becomes rw = 1.078, slightly greater than 1, which means that net drag reduction is not obtained. On the other hand, in the ballast condition (empty condition), the draft becomes d = 12m, then rw = 0.979, approximately 2% net reduction being obtained. This means that, using the present level of technology, a tanker that runs between Japan and the Middle East can get net drag reduction one way. If the amount of air can be economized to half by improving techniques, the net work ratio becomes rW = 0.952 even at full load condition of d = 20m. In general, in order for an energy-saving effect not to be embedded into measurement errors, it has to be 5% at least, and therefore, this much of net drag reduction is needed. This rough estimation suggests that, in order to put microbubbles into practical use, it is necessary to improve drag reduction efficiency at least twice as much and/or to combine the technique with other efforts such as developing a new hull form suited for microbubbles like one with very shallow draft and very wide flat bottom. Also it is necessary to take measures to prevent fouling at air injection, and to estimate influence on noise and vibration problems, and to prevent them if necessary.
Conclusion
Various numerical simulation techniques were used to elucidate the mechanism of the multiple drag reduction. The techniques can be grouped by the ways of bubbles turbulence are modeled. For investigating the smaller scale interaction between bubbles and turbulence DNS methods can be used. DNS has been applied to a laminar 3-Dimensional low Reynolds number turbulent channel flow in this study. It has been seen that laminar interactions and the effect of compressibility of microbubbles cannot explain the decrease in the frictional drag.
From the results presented here, and other simulation results obtained, certain observations may be made.
Â¢ The near-wall concentration of the bubbles is an important factor and when this drops to a low level the drag reduction effect is diminished or eliminated. This indicates that the interactions of the bubbles with the near-wall turbulence are critical.
Â¢ Smaller bubbles in general are more effective in reducing drag.
Â¢ At higher Reynolds numbers the range of bubbles sizes may be effective in reducing drag appears to be broader.
The effects of the bubble size on the bubble dispersion and the skin friction reduction have been studied experimentally. The main conclusion is that small bubbles possibly decrease the overall efficiency of the microbubbles drag reduction through dispersion. This effect of the bubble size on the dispersion is significant when the average bubble diameter is smaller than 0.5mm, and in the spatially developing boundary generator such as the slit plate used in the experiment using a 50m flat ship is suitable in practical implementation.
References
1) McCormick, M.E. and Bhattacharyya, R., 1973, Drag Reduction of a Submersible Hull by Electrolysis, Naval Engineers Journal, Vol.85, No.2, pp. 11-16.
2) C. Merkle,. and S. Deutsch:"Drag Reduction in Liquid Boundary Layers by Gas Injection", Progress in Astronautics and Aeronautics vol.123, AIAA.
3) Guin M.M., Kato, H., Maeda, M., and Miyanaga, M., 1996, Reduction of Skin Friction by Microbubbles and Its Relation with Near-Wall Bubble Concentration in a Channel, Journal of Marine Science and Technology, Vol. 1, No. 5, pp. 241-254.
4) Takahashi,T. et al.,1997,"Streamwise Distribution of the Skin Friction Reduction by Microbubbles", J. of the Society of Naval Architects of Japan, vol.182, pp.1-8.
5) Y. Kodama,et al.,:Experimental study on microbubbles and their applicability to ships for skin friction reduction., International Journal of Heat and Fluid Flow21 ,2000.
6) Kato, H., Iwashita, T., Miyanaga, M., Yamaguchi, H, 1998, Effect of Microbubble Cluster on Turbulent Flow Structure, IUTAM Symp. onMechanics of Passive and Active Flow Control,Gottingen, Germany, pp.1-6.
7) Watanabe, O. et al.,1998,"Measurements of Drag Reduction by Microbubbles Using Very Long Ship Models", J. of Soc. Naval Architects, Japan, vol.183, pp.53-63.
8) T. Takahashi et al.:Experimental Study on Drag Reduction by Microbubbles Using a 50m-long Flat Plate Ship", 2nd International Symposium on Turbulence and Shear Flow Phenomena (TSFP-2), June, 2001.
9) Madavan, N.K. et al., Reduction of turbulent skin friction by microbubbles, Physics of fluids, vol.27, No.2, pp.356-363, 1984.
10) M.R Maxey, J. Xu, S. Dong and G.E Karniadakis, Centre for Fluid Mechanics,
Turbulence and Computation division of Applied Mathematics, Brown University, Providence, USA.
11) Sugiyama, K., Takagi, S. and Matsumoto, Y. (2001)J.Soc. Naval Architecture Japan
CONTENTS
1) INTRODUCTION 1
2) MECHANISM AND SCALE EFFECT OF SKIN FRICTION 4
BY MICRO BUBBLES
3) VARIOUS METHODS OF BUBBLE GENERATION 9
4) FRICTIONAL DRAG REDUCTION BY MICRO BUBBLES 15
5) APPLICATION OF MICROBUBBLES IN SHIPS 16
6) ISSUES ON THE APPLICATION OF MICROBUBBLES TO SHIPS 19
7) NET DRAG REDUCTION BY MICRO BUBBLES 23
8) HOW TO ICREASE DRAG REDUCTION EFFECT 26
9) DRAG REDUCTION ESTIMATION IN CASE OF TANKERS 26
10) CONCLUSION 28
11) REFERENCES 29
Post: #2
ABSTRACT

DRAG REDUCTION
Whatâ„¢s Drag?
Drag is a mechanical force generated by a solid object moving through a fluid.
Forces Acting are :-
Frictional resistance
Wave making resistance
Air resistance.

Whatâ„¢s Microbubble?
Microbubbles is a drag reduction device that reduces skin friction of a solid body moving in water by injecting small bubbles into the turbulent boundary layer developing on the solid body.

Applications
Itâ„¢s application is confined to the ships, especially large ships. Ships such as tankers play a major role in marine transportation.

Conclusion
The near-wall concentration of the bubbles is an important factor and when this drops to a low level the drag reduction effect is diminished or eliminated. This indicates that the interactions of the bubbles with the near-wall turbulence are critical.
Smaller bubbles in general are more effective in reducing drag.
At higher Reynolds numbers the range of bubbles sizes may be effective in reducing drag.
Post: #3
MICROBUBBLE TECHNOLOGY

Presented by

PRASANTH C
NO: 39
S 7 MECHANICAL
[attachment=8235]

Introduction

Two types of Drag – Friction and Pressure Drag
Friction Drag due to friction
Pressure Drag due to Eddying motions
Stream line body –Viscous Drag
Bluff body – Pressure Drag
hy Microbubbles

Mechanisms

Local void ratio

Injection point

Boundary layer thickness

Methods for bubble generation

Venturi tube type

Tangential water jet

Foaming of dissolved air

Pros and Cons

large reduction in skin friction up to 80%

No pollutions

Can apply any surface orientation

Affected less by roll and pitch

large amount of air (order of 1000cubic metre per min)
Applications

Power saving in VLCC

Pipelines
Conclusion

small bubbles possibly decrease the overall efficiency of the microbubble drag reduction through dispersion

The understanding of the mechanism of turbulence intensity reduction by microbubbles is the key for break-through.
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